Obviously Sebastian Shaw couldn't play Anakin, but they could have found somebody appropriate for the role. Shaw left his mark on the character, but the character wasn't as locked to Shaw as Spock was to Nimoy. As such it would have been perfectly ok to have a younger actor to play the character if they'd gotten somebody appropriate. But they didn't. We always knew a younger actor would have to play Anakin, that was the deal from the start, way back in the days the OOT was coming out. Whereas Spock was just Nimoy, plain and simple, defined by him to a great degree and played by him in endless hours of tv/film, very different from playing one scene, however significant.
As it is, there was a lot more wrong with the prequels than just the casting of Anakin, but the awful writing and acting for Anakin was one of the worst things about it. A better actor, with different writing and directing could have turned out an Anakin that worked. But Spock IS Nimoy and it's not ok to give the role to someone else for more than flashback scenes and little stuff like that. The planned Star Wars prequels (which would have been different from the ones we got), with their younger version of Anakin played by a younger actor, were part of the planned Star Wars saga long before the OT was completed and before Shaw got the role. But this Star Trek movie wasn't on the horizion until recent times when they decided to go back and fuck up the old characters wth bad imitations. There's a marked difference in the Anakin and Spock situations.
However it WASN'T ok to give the Anakin role to Hayden Christensen, who fucked it up totally. And similarly, Quinto gets Spock totally wrong. Quinto's Spock is an insult to the character and the fans of the character, just as Hayden's Anakin was an insult to Vader/Anakin and to Star wars fans. It would have been better to have no prequels than ones in which the character of Anakin was so ruined. And it would have been better to have no new Trek than to have the character of Spock crapped on like that.